1. Field of the Invention
The present invention relates to a light-emitting polymer composition and an organic electroluminescent (EL) display device using the same.
2. Description of the Related Art
An organic EL display device includes an anode, a hole injection layer, a hole transportation layer, a light-emitting layer, an electron transporting layer, an electron injection layer, and a cathode, which are sequentially stacked on a substrate.
The above-described layers of the organic EL display device are made of a low-molecular organic material or a high-molecular organic material (i.e., a polymer). The layers of the organic EL display device comprising a low-molecular organic material are formed by, for example, a vacuum deposition technique, while the layers of the organic EL display comprising a high-molecular organic material are formed by, for example, a spin coating technique.
In general, an organic EL display device having a light-emitting layer of one color made of a high-molecular organic material is easy to manufacture and is lower in driving voltage than that of a low-molecular organic material. However, an organic EL display device having a light-emitting layer of one color made of the high-molecular organic material is lower in light-emitting efficiency and shorter in life span than that of the low-molecular organic material.
On the other hand, in forming a light-emitting layer of a full color, an organic EL display device having the high-molecular organic material has difficulties in patterning red, green and blue light-emitting layers using an ink jet technique or a laser transfer technique, leading to a low light-emitting efficiency and a short life span.
Conventional light-emitting polymer materials are patterned by the ink-jet technique or the laser transfer technique. However, in most cases, light-emitting polymer materials are not transferred using the laser transfer technique, which is a kind of a thermal transfer technique.
The thermal transfer technique requires at least a light source, a transfer film, and a substrate. Light emitted from the light source is absorbed into a light absorbing layer of the transfer film and then converted into heat energy. An image forming material of the transfer film is transferred to the substrate by the heat energy, thereby forming a desired image on the substrate. The thermal transfer technique is also used to form a color filter of a liquid crystal display (LCD) device.
FIG. 1 shows a schematic view illustrating a laser transfer operation for patterning a light-emitting layer of a conventional organic EL display device.
Referring to FIG. 1, an organic film S2 is formed on a substrate S1. A laser beam is irradiated to the substrate S1 to separate the organic film S2 from the substrate S1 and transfer it to a substrate S3.
Here, parameters that determine a transfer characteristic include an adhesion W12 between the substrate S1 and the organic film S2, a cohesion W22 between elements of the organic films S2, and an adhesion W23 between the organic film S2 and the substrate S3.
The adhesion W12 and W23 and the cohesion W22 can be described by a surface tension and an interfacial tension as follows:W12=γ1+γ2−γ12;W22=2γ22; andW23=γ2+γ3−γ23,
where γ1 denotes a surface tension of the substrate S1, γ2 denotes a surface tension of the organic film S2, γ3 denotes a surface tension of the substrate S3, γ12 denotes an interfacial tension between the substrate S1 and the substrate S2, γ22 denotes an interfacial tension between the elements of the organic film S2 and γ23 denotes an interfacial tension between the substrate S2 and the substrate S3.
As the cohesion between the elements of the organic film S2 becomes smaller than the adhesion between the respective substrates S1 and S3 and the organic film S2, the laser transfer characteristic improves.
However, a lighting-emitting layer of the conventional organic EL display device is usually made of a polymer film which has a high molecular weight. Therefore, the cohesion between elements of the polymer film is relatively large. Accordingly, the polymer film shows a bad transfer characteristic in the conventional organic EL display device.
That is, the conventional art does not disclose a technique that can improve a transfer characteristic where the light-emitting layer is formed using the laser transfer technique.